To make zebra cake - divide cake batter in half. Tint each half of the batter with the desired color.
Grease and flour two 8-inch cake pans.
Using a medium cookie scoop or spoon, drop approximately 1 tablespoon of one color of batter in the middle of each cake pan. Top the batter with 1 tablespoon of the alternate color of batter. Repeat this process until all batter is used. (you do not have to wait for the batter to spread before adding the alternate color)
Bake cake according to package directions. Cool completely.
In the bowl of an electric mixer fitted with the paddle attachment, beat butter until smooth and creamy, 2 to 3 minutes. With the mixer on low speed, add chocolate until just combined. Add cream, vanilla, and sugar; mix on medium until well combined. Add black food coloring if desired.
Frost cake with chocolate buttercream. Cut Twizzler's to size - about 1/2-inch taller than cake. Press Twizzlers on edge of the cake. Top cake with Reese's pieces.